Owing to their good impact resistance, heat resistance and electric characteristics, among others, thermoplastic resins, in particular polycarbonate resins, are widely used as materials of electric and electronic parts, OA (office automation) apparatus and instruments, and household utensils, or as building materials. Polycarbonate resins, though higher in flame retardancy as compared with polystyrene and other resins, are required to be highly flame retardant in particular in such fields as electric and electronic parts, OA apparatus and instruments and the like and, therefore, attempts have been made to improve their flame retardancy by adding various flame retardants. Thus, for instance, the addition of organohalogen compounds or organophosphorus compounds has so far been in wide practice. However, organohalogen compounds and organophosphorus compounds have a problem from the toxicity viewpoint. In particular, it is a drawback of organohalogen compounds that they generate a corrosive gas upon combustion thereof. Thus, the demand for halogen-free and phosphorus-free flame retardants has been increasing in recent years.
The utilization of polyorganosiloxane compounds (also called silicones) as halogen-free and phosphorus-free flame retardants has been proposed. For example, Japanese Kokai Publication Sho-54-36365 describes that kneading of a monoorganopolysiloxane-based silicone resin with a non-silicone polymer gives a flame retardant resin.
Japanese Kohyo Publication Hei-3-48947 describes that a mixture of a silicone resin and a salt of a metal of the group IIA provides thermoplastic resins with flame retardancy.
Japanese Kokai Publication Hei-8-113712 describes a method of producing flame retardant resin compositions which comprises dispersing a silicone resin prepared by blending 100 parts by weight of a polyorganosiloxane with 10 to 150 parts by weight of a silica filler in thermoplastic resins.
Japanese Kokai Publication Hei-10-139964 describes that flame retardant resin compositions are obtained by adding a solvent-soluble silicone resin having a weight average molecular weight of not less than 10,000 but not more than 270,000 to an aromatic ring-containing non-silicone resin.
However, the silicone resins described in the above-cited publications are indeed effective in providing flame retardancy but their effects are still unsatisfactory. When the addition level is increased to fill up the shortage, a problem arises that the impact resistance of the resin composition decreases, making it difficult to obtain flame retardant resin composition balanced between flame retardancy and impact resistance.
Japanese Kokai Publication 2000-17029 describes that when a composite rubber-based flame retardant produced by graft polymerization of a vinyl monomer onto a composite rubber composed of a polyorganosiloxane rubber and a polyalkyl (meth)acrylate rubber is incorporated in thermoplastic resins, flame retardant resin compositions can be obtained.
Japanese Kokai Publication 2000-226420 describes that flame retardant resin compositions can be obtained by incorporating a polyorganosiloxane-based flame retardant produced by grafting a vinyl monomer onto composite particles consisting of an aromatic group-containing polyorganosiloxane and a vinyl polymer in thermoplastic resins.
Japanese Kokai Publication 2000-264935 describes that flame retardant resin compositions can be obtained by incorporating, in thermoplastic resins, a polyorganosiloxane-containing graft copolymer prepared by graft copolymerization of a vinyl monomer onto polyorganosiloxane particles not larger than 0.2 μm in size.
The flame retardant resin compositions described in the above-cited Japanese Kokai Publication 2000-17029, Japanese Kokai Publication 2000-226420 and Japanese Kokai Publication 2000-264935 all indeed show satisfactory levels of impact resistance but are unsatisfactory in flame retardancy. Thus, they have a problem that the flame retardancy-impact resistance balance is poor.